It is known to use different materials, or stacking of materials, to produce kitchen utensils: steel (alloyed or not), aluminum, stainless steel (that is to say, generally containing more than 11% chromium), copper or silver alloys in particular, with or without surface coatings such as polymer layers based on polytetrafluoroethylene (PTFE, distributed in particular under the trademark Teflon). Each material has its own advantages and drawbacks for this type of application.
Aluminum is very resistant to corrosion during steps of washing the utensils, including in a dishwasher with detergents, but on the other hand can easily be scratched and its non-stick properties are mediocre. For this reason it is often associated with a coating of polytetrafluoroethylene type.
Austenitic stainless steel (containing approximately 18% chromium and 10% nickel) also has good corrosion resistance and slightly better scratch resistance than aluminum. On the other hand, it is a poor heat conductor which does not facilitate the homogenization of temperature for cooking utensils such as woks, frying pans, griddles, casserole dishes, pots, broiler plates, fryers, grills (barbecues), molds or saucepans.
Copper is a very good heat conductor which is recognized for providing good quality cooking. However, it is an expensive material reserved for top of the range utensils.
Non-stainless steels have a great advantage over all the other aforementioned materials, which is their price. To be precise, steels, especially unalloyed steels (those without any added component) or weakly alloyed steels (that is to say in which no added component exceeds 5% by weight), are easily and abundantly available, their price is low and varies little relative to that of stainless steels or copper. It is for this reason that non-stainless steels are very widely used as the basic material for bottom of the range utensils.
However, these steels have very low corrosion resistance, especially on cleaning the utensils with detergents (washing in a dishwasher is ruled out), their surface is easily scratched and their non-stick properties are mediocre.
The teaching of patent US 2008/0118763 A1 shows that ferritic nitrocarburizing may be applied to kitchen utensils at a temperature of 1060° F. (571° C.) for 3 h in an atmosphere of 55% nitrogen, 41% ammonia and 4% CO2. Carried out next are gaseous oxidation (post-oxidation) at a temperature of 800° F. (˜427° C.) and temporary protection by baking at 500° F. (260° C.) for 45 minutes using a cooking oil. According to this document, the treated surfaces have increased hardness and improved corrosion resistance.
The treatments of nitriding, nitrocarburizing, oxinitriding and oxinitrocarburizing (the prefix oxi- means that after the nitriding or nitrocarburizing, an oxidizing step is carried out) are used in the mechanical industry (in the automotive sector: valves, gas struts, ball joints; in construction equipment: articulations, hydraulic jacks, etc.)
These treatments are carried out industrially either using gas (ammonia-based atmospheres) or using plasma (glow discharge at low pressure), or using liquid (ionic liquid media, see for example the document US2003084963).
In industry, the treatments of nitriding, nitrocarburizing, oxinitriding and oxinitrocarburizing are conventionally carried out in the ferritic phase (in the iron-nitrogen diagram), that is to say at temperatures less than 592° C.
A layer of iron nitride is formed, and the layer below is referred to as diffusion layer.
Beyond 592° C., the γN phase forms (nitrogen-containing austenite, generally named γN) between the nitride layer and the diffusion layer. Nitrogen-containing austenite is a microstructure that is particular to steel. The precise temperature beyond which the γN phase forms depends on the exact composition of the steel. If the latter contains a lot of alloying components, this temperature limit value may shift up to 600° C.
This nitrogen-containing austenite layer transforms into nitrogen-containing braunite, another microstructure particular to steel, under the effect of temperature at the oxidation step which is conventionally carried out after the nitriding or nitrocarburizing step. However, in the field of mechanical parts, the oxidation step is generally carried out since it is desired that the parts be corrosion resistant, nitriding increasing wear resistance and the oxidation increasing the corrosion resistance.
This retransformation into braunite is not generally desired since for the mechanical applications for which nitrocarburizing is generally destined, the presence of a nitrogen-containing braunite layer gives rise to fragility in case of impact.
More particularly, the typical mechanical stresses whose effect it is usually sought to limit by nitrocarburizing are cyclic stresses and/or alternating stresses which will recur with a high number of cycles, such as for example superficial fatigue or impact.
The presence of a braunite layer is thus generally ruled out since the fragility of that layer may lead to flaking or splitting of the nitride layer under the effect of impact (high energy transfer which is brief and localized between two parts moving relative to each other). Nitrocarburizing and nitriding are thus conventionally carried out in ferritic phase. When austenitic nitriding is carried out, the post-oxidation step is then generally performed at a temperature below 200° C. to avoid the retransformation of the nitrogen-containing austenite into braunite (see for example patent EP1180552).
Furthermore, with regard to the teaching of patent US 2008/0118763 A1, the applicant has noted that at the post-oxidation step carried out just after the nitrocarburizing, the high temperatures used (greater than 200° C.) give rise to tempering in the diffusion zone. The consequence of this annealing is a drop in the hardness of the diffusion zone which is detrimental to the scratch resistance of the kitchen utensils treated.
Consequently, when a load is applied which stresses the material in its core and not just the hard surface layer, the substrate deforms and the hard surface layer splits and flakes.
The same applies to the step of baking the temporary protective agent which is carried out between 150 and 260° C., as well as during the life of the utensils, at each utilization above 200° C. of those kitchen utensils.
This is in particular detrimental in the case of the low carbon steels which are generally used for kitchen utensils.
It is moreover to be noted that nitrocarburizing methods require a high energy input, and that it is desirable to control the treatment time, to limit the final costs. One of the drawbacks of the treatment range presented by document US 2008/0118763 A1 is its duration which is long (3 hours).
In this context, the problem which the invention sets out to solve is to give improved non-stick, scratch resistant and corrosion resistant properties to the surface of kitchen utensils made of steel (not alloyed or weakly alloyed), with improved production costs.